1. Field of the Invention
The present invention relates generally to synchronization wave detecting circuits. More particularly, the invention is directed to an improved keyed type synchronization wave detecting circuit that is highly suitable for use as a video wave detecting circuit of a color television receiver.
2. Description of Related Art
There are two main types of synchronization wave detecting circuits. The first of these types is a so called "sequential" system which carries out synchronizing operation and wave detection by using a signal for wave detection that is sequentially synchronized with a carrier of a received signal. The second of these types is a so called "keyed" system which carries out synchronizing operation and wave detection using a signal for wave detection synchronized with the carrier every predetermined short interval.
The circuit arrangement of a sequential-type system is relatively simple, however in operation it tends to become unstable. Signal distortion can occur during wave detection when the voltage level of the received signal changes and when noise and/or ghost signals are superimposed on the received signal.
The use of a keyed-type system overcomes, to a certain extent (not completely), the problems associated with the operation of the sequential-type system. The keyed-type system can be operated in a manner such that the signal for wave detection is phase-locked to the carrier of the received signal. However, the operation of the wave detecting circuit occasionally tends to become unstable until the signal for wave detection is phase-locked to the carrier of the received signal.
In particular, when the voltage level of the received signal is varied and/or the waveform of the received signal changes by passing through a nonlinear circuit, the operation of the wave detecting circuit occasionally tends to become unstable. Such a problem is not negligible and must be overcome to achieve practical operation.